The present invention relates to a high frequency electrical shield assembly for housing an electrical circuit such as a television tuner, cable television converter or the like.
A known construction of a high frequency shield assembly comprises an electrically conductive box having one open side. An electrically conductive inner plate engages with the covers the open side. A resilient sheet is placed over the inner plate and an outer plate is spring biased into engagement with the resilient sheet to firmly hold the inner plate in contact with the box. The resilient sheet is formed of a material such as rubber or polyethylene having a high dielectric constant, and serves the purpose of maintaining the inner plate in firm engagement with the box when the shield assembly is exposed to vibration or impact.
This prior art shield assembly suffers from the drawback that it comprises two metal plates and is therefore relatively expensive to manufacture. It suffers from a further drawback in that the two plates act as a wagequide at the operating frequencies of the circuit housed in the shield. This causes dissipation of radio frequency energy from the circuit in the shield. In a case in which the circuit is a television tuner comprising a local oscillator, the propogation of radio frequency through the resilient material between the plates causes instability in the frequency of the local oscillator.